Comet (programming) - Alternatives

Alternatives

Browser-native technologies are inherent in the term Comet. Attempts to improve non-polling HTTP communication have come from multiple sides:

  • The HTML 5 draft specification produced by the Web Hypertext Application Technology Working Group (WHATWG) specifies so called server-sent events, which defines a new Javascript interface EventSource and a new MIME type text/event-stream. Experimental implementation of this feature was introduced in Opera 9.
  • The HTML 5 WebSocket API working draft specifies a method for creating a persistent connection with a server and receiving messages via an onmessage callback.
  • The Bayeux protocol by the Dojo Foundation. It leaves browser-specific transports in place, and defines a higher-level protocol for communication between browser and server, with the aim of allowing re-use of client-side JavaScript code with multiple Comet servers, and allowing the same Comet server to communicate with multiple client-side JavaScript implementations. Bayeux is based on a publish/subscribe model, so servers supporting Bayeux have publish/subscribe built-in.
  • The BOSH protocol by the XMPP standards foundation. It emulates a bidirectional stream between browser and server by using two synchronous HTTP connections.
  • The JSONRequest object, proposed by Douglas Crockford, would be an alternative to the XHR object.
  • Use of plugins, such as Java applets or the proprietary Adobe Flash (using RTMP protocol for data streaming to Flash applications). These have the advantage of working identically across all browsers with the appropriate plugin installed and need not rely on HTTP connections, but the disadvantage of requiring the plugin to be installed
  • Google announced a new Channel API for Google App Engine, implementing a Comet-like API with the help of a client Javascript library on the browser. It could be later replaced by HTML5 WebSocket, however.

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